The present invention relates generally to methods of biaxially stretching films and such films, and more particularly to methods of stretching films in two directions simultaneously and such films.
It has been known in the art to biaxially stretch films. Additionally, several methods and apparatuses have been described for biaxially stretching films simultaneously in two directions. See, e.g., U.S. Pat. Nos. 2,618,012; 3,046,599; 3,502,766; 3,890,421; 4,330,499; 4,525,317; and 4,853,602. The variability in stretch profiles available with some of these methods and apparatus has also been described.
For example, U.S. Pat. No. 3,890,421 illustrates in its FIG. 1 what the text describes as: Curve I representing normal sequential drawing with lateral drawing following longitudinal drawing; Curve II corresponding to reverse sequential drawing with longitudinal drawing following transverse drawing; and diagonal Curve II (sic, Curve III) representing a regularly progressive simultaneous biaxial drawing in both lateral and longitudinal directions. The ""421 patent also states that simultaneous drawing can be performed along an indefinite number of curves between curves I and II with the methods and apparatus described therein (column 4, lines 14-31). Without providing detailed descriptions of stretch profiles to achieve the stated objects, the ""421 patent states that the object of the method and apparatus described therein is to regulate the resistance, tensile strength, modulus of elasticity, shrinkage, and flatness of biaxially drawn film by controlling drawing and slack tension throughout the drawing process while avoiding the limiting factors from successive biaxial drawing (column 3, lines 34-39).
U.S. Pat. No. 4,853,602 states that with the method and apparatus described therein, sequential drawing may be performed with lateral preceding longitudinal or with longitudinal preceding lateral (column 34, lines 35-55). This patent also states that for simultaneous stretching, any desired drawing of the film can be achieved (column 35, lines 17 et seq.).
Stretch profiles which include relaxing the film in one or more directions after achieving a higher intermediate stretch are also known. For example, U.S. Pat. No. 4,330,499 states that shrinking of the film occurs in the longitudinal direction at up to 10% of the previous produced longitudinal stretching, over the last 5 to 10% of the stretch apparatus length, preferably while the film is further stretched in the transverse direction (see Abstract).
Uniform thickness is important in adhesive tape manufacturing because it is an indication of the uniformity of the film properties and because non-uniform thickness leads to gapping or telescoping of tape rolls.
The majority of commercially available biaxially oriented polypropylene films are produced by the flat film or tenter stretching process. Typical tenter processes serve to biaxially stretch films either predominately simultaneously or predominately sequentially. Currently, simultaneously tenter stretched films comprise a minor part of the film backing market because, although such processes can continuously stretch films in both longitudinal and transverse directions, they have historically proven costly, slow, and inflexible regarding allowable stretching ratios.
One aspect of the present invention provides a method of biaxially stretching a polymeric film. The method comprises the steps of:
a) imparting a sufficiently high temperature to the film to allow a significant amount of biaxial stretch; and
b) biaxial tenter stretching the film to a final first direction stretch parameter and a final second direction stretch parameter, wherein at least 75% of the final first direction stretch parameter is attained before no more than 50% of the final second direction stretch parameter is attained, and wherein the final first direction stretch parameter is no greater than the final second direction stretch parameter.
In one preferred embodiment of the above method of claim 1, step b) comprises biaxial tenter stretching the film such that a substantial portion of the first direction stretch and the second direction stretch is performed simultaneously.
In another preferred embodiment of the above method, at least 90% of the final first direction stretch parameter is attained before no more than 50% of the final second direction stretch parameter is attained.
In another preferred embodiment of the above method, the first direction is the MD and the second direction is the TD.
In another preferred embodiment of the above method, the final first direction stretch parameter is less than the natural stretch parameter for a proportional stretch profile.
In another preferred embodiment of the above method, the final first direction stretch parameter is less than the uniaxial natural stretch parameter.
In another preferred embodiment of the above method, the final second direction stretch parameter is greater than the natural stretch parameter for a proportional stretch profile.
In another preferred embodiment of the above method, the final second direction stretch parameter is greater than the uniaxial natural stretch parameter.
In another preferred embodiment of the above method, the film comprises a thermoplastic film. More preferably, the film comprises a semi-crystalline film. Still more preferably, the film comprises a polyolefin. In a particularly preferred embodiment, the film comprises polypropylene.
In another preferred embodiment of the above method, step b) further comprises grasping the film with a plurality of clips along the opposing edges of the film and propelling the clips at varying speeds in the machine direction along clip guide means that diverge in the transverse direction.
In another preferred embodiment of the above method, step b) further includes stretching the film to more than 100% of the final first direction stretch parameter before no more than 50% of the final second direction stretch parameter is attained, and thereafter retracting the film in the machine direction to the final first direction stretch parameter. A significant portion of the retraction may be performed simultaneously with a portion of the second direction stretch.
In another preferred embodiment of the above method, step b) further includes stretching the film to a peak first direction stretch parameter that is at least 1.2 times the final first direction stretch parameter, and thereafter retracting the film in the first direction to the final first direction stretch parameter. A significant portion of the retraction may be performed simultaneously with a portion of the second direction stretch. Furthermore, step b) further may include stretching the film to the peak first direction stretch parameter before no more than 50% of the final second direction stretch parameter is attained.
In another aspect, the present invention provides a method of biaxially stretching a polypropylene film. The method comprising the steps of: a) imparting a sufficiently high temperature to the film to allow a significant amount of biaxial stretch; and b) biaxial tenter stretching the film to a final first direction stretch parameter and a final second direction stretch parameter. In such a method: i) a substantial portion of the first direction stretch and second direction stretch is performed simultaneously; ii) at least 90% of the final first direction stretch parameter is attained before no more than 50% of the final second direction stretch parameter is attained; iii) the final first direction stretch parameter is not greater than the final second direction stretch parameter; and iv) the final first direction stretch parameter less than the natural stretch parameter for a proportional stretch profile.
In a further aspect, the present invention provides another method of biaxially stretching a polymeric film. The method comprising the steps of: a) imparting a sufficiently high temperature to the film to allow a significant amount of biaxial stretch; and b) biaxial tenter stretching the film according to a stretch profile to a final first direction stretch parameter and a final second direction stretch parameter, wherein the final first direction stretch parameter is no greater than the final second direction stretch parameter. In such a method: i) a straight line between the point defining zero stretch parameter and the point defining the final first and second direction stretch parameters represents a proportional stretch profile and defines a proportional stretch area; and ii) the curve representing the stretch profile between the point defining zero stretch parameter and the point defining the final first and second direction stretch parameters defines an area at least 1.4 times the proportional stretch area.
In one preferred embodiment of the above method, step b) comprises stretching the film such that the curve representing the stretch profile between the point defining zero stretch parameter and the point defining the final first and second direction stretch parameters defines an area at least 1.7 times the proportional stretch area.
In another preferred embodiment of the above method, step b) comprises stretching the film such that a substantial portion of the first direction stretch and second direction stretch is performed simultaneously.
In another preferred embodiment of the above method, the first direction is the MD and the second direction is the TD.
In another preferred embodiment of the above method, step b) comprises stretching the film to a final first direction stretch parameter less than the natural stretch parameter for a proportional stretch profile.
In another preferred embodiment of the above method, step b) comprise stretching the film to a final first direction stretch parameter less than the uniaxial natural stretch parameter.
In another preferred embodiment of the above method, the final second direction stretch parameter is greater than the natural stretch parameter for a proportional stretch profile.
In another preferred embodiment of the above method, the final second direction stretch parameter is greater than the uniaxial natural stretch parameter.
In another preferred embodiment of the above method, the film comprises a thermoplastic film. More preferably, the film comprises a semi-crystalline film. Still more preferably, the film comprises a polyolefin. In a particularly preferred embodiment, the film comprises polypropylene.
In another preferred embodiment of the above method, step b) further comprises grasping the film with a plurality of clips along the opposing edges of the film and propelling the clips in the machine direction along clip guide means that diverge in the transverse direction.
In another preferred embodiment of the above method, step b) further includes stretching the film to more than 100% of the final first direction stretch parameter before no more than 50% of the final second direction stretch parameter is attained and thereafter retracting the film in the first direction to the final machine direction stretch parameter. A significant portion of the retraction may be performed simultaneously with a portion of the second direction stretch.
In another preferred embodiment of the above method, step b) further includes stretching the film to a peak first direction stretch parameter that is at least 1.2 times the final first direction stretch parameter, and thereafter retracting the film in the first direction to the final first direction stretch parameter. A significant portion of the retraction may be performed simultaneously with a portion of the second direction stretch.
In another preferred embodiment of the above method, step b) further includes stretching the film to the peak first direction stretch parameter before no more than 50% of the final second direction stretch parameter is attained.
In yet another aspect, the present invention provides a method of biaxially stretching a polypropylene film. The method comprising the steps of: a) imparting a sufficiently high temperature to the film to allow a significant amount of biaxial stretch; and b) biaxial tenter stretching the film according to a stretch profile to a final first direction stretch parameter and a final second direction stretch parameter. In such a method: i) a substantial portion of the first direction stretch and second direction stretch is performed simultaneously; ii) a straight line between the point defining zero stretch parameter and the point defining the final first and second direction stretch parameters represents a proportional stretch profile and defines a proportional stretch area; and iii) the curve representing the stretch profile between the point defining zero stretch parameter and the point defining the final first and second direction stretch parameters defines an area at least 1.4 times the proportional stretch area; iv) the final first direction stretch parameter is no greater than the final second direction stretch parameter; and v) the final first direction stretch parameter is less than the natural stretch parameter for a proportional stretch profile.
The present invention also provides a film obtained by any of the methods described above. The present invention also provides a tape comprising a backing including a fist major surface and a layer of adhesive on said first major surface, wherein said backing comprises a the film a film obtained by any of the methods described above.
Certain terms are used in the description and the claims that, while for the most part are well known, may require some explanation. xe2x80x9cBiaxially stretched,xe2x80x9d when used herein to describe a film, indicates that the film has been stretched in two different directions, a first direction and a second direction, in the plane of the film. Typically, but not always, the two directions are substantially perpendicular and are in the machine direction (xe2x80x9cMDxe2x80x9d) of the film and the transverse direction (xe2x80x9cTDxe2x80x9d) of the film. Biaxially stretched films may be sequentially stretched, simultaneously stretched, or stretched by some combination of simultaneous and sequential stretching. xe2x80x9cSimultaneously biaxially stretched,xe2x80x9d when used herein to describe a film, indicates that significant portions of the stretching in each of the two directions are performed simultaneously. Unless context requires otherwise, the terms xe2x80x9corient,xe2x80x9d xe2x80x9cdraw,xe2x80x9d and xe2x80x9cstretchxe2x80x9d are used interchangeably throughout, as are the terms xe2x80x9coriented,xe2x80x9d xe2x80x9cdrawn,xe2x80x9d and xe2x80x9cstretched,xe2x80x9d and the terms xe2x80x9corienting,xe2x80x9d xe2x80x9cdrawing,xe2x80x9d and xe2x80x9cstretching.xe2x80x9d
The term xe2x80x9cstretch ratio,xe2x80x9d as used herein to describe a method of stretching or a stretched film, indicates the ratio of a linear dimension of a given portion of a stretched film to the linear dimension of the same portion prior to stretching. For example, in a stretched film having an MD stretch ratio (xe2x80x9cMDRxe2x80x9d) of 5:1, a given portion of unstretched film having a 1 cm linear measurement in the machine direction would have 5 cm measurement in the machine direction after stretch. In a stretched film having a TD stretch ratio (xe2x80x9cTDRxe2x80x9d) of 5: 1, a given portion of unstretched film having a 1 cm linear measurement in the transverse direction would have 5 cm measurement in the transverse direction after stretch.
xe2x80x9cArea stretch ratio,xe2x80x9d as used herein, indicates the ratio of the area of a given portion of a stretched film to the area of the same portion prior to stretching. For example, in a biaxially stretched film having an overall area stretch ratio of 50:1, a given 1 cm2 portion of unstretched film would have an area of 50 cm2 after stretch.
The mechanical stretch ratio, also know as nominal stretch ratio, is determined by the unstretched and stretched dimensions of the overall film, and can typically be measured at the film grippers at the edges of the film used to stretch the film in the particular apparatus being used. Global stretch ratio, refers to the overall draw ratio of the film after the portions that lie near the grippers, and thus are affected during stretching by the presence of the grippers, have been removed from consideration. The global stretch ratio can be equivalent to the mechanical stretch ratio when the input unstretched film has a constant thickness across its full width and when the effects of proximity to the grippers upon stretching are small. More typically, however, the thickness of the input unstretched film is adjusted so as to be thicker or thinner near the grippers than at the center of the film. When this is the case, the global stretch ratio will differ from the mechanical or nominal stretch ratio. These global or mechanical ratios are both to be distinguished from a local stretch ratio. The local stretch ratio is determined by measuring a particular portion of the film (for example a 1 cm portion) before and after stretch. When stretch is not uniform over substantially the entire edge-trimmed film, then the local ratio can be different from the global ratio. When stretch is substantially uniform over substantially the entire film (excluding the area immediately near the edges and surrounding the grippers along the edges), then the local ratio will be substantially equal to the global ratio. Unless the context requires otherwise, the terms first direction stretch ratio, second direction stretch ratio, MD stretch ratio, TD stretch ratio, and area stretch ratio are used herein to describe the global stretch ratio.
The term xe2x80x9cstretch parameterxe2x80x9d is used to indicate the value of the stretch ratio minus 1. For example xe2x80x9cfirst direction stretch parameterxe2x80x9d and xe2x80x9csecond direction stretch parameterxe2x80x9d are used herein to indicate the value of first direction stretch ratio minus 1, and second direction stretch ratio minus 1, respectively. Likewise, the terms xe2x80x9cMD stretch parameterxe2x80x9d and xe2x80x9cTD stretch parameterxe2x80x9d are used herein to indicate the value of MD stretch ratio minus 1, and TD stretch ratio minus 1, respectively. For example, a film that has not been stretched in the machine direction would have an MD stretch ratio of 1 (i.e., dimension after stretch is equal to dimension before stretch). Such a film would have an MD stretch parameter of 1 minus 1, or zero (i.e., the film has not been stretched). Likewise, a film having an MD stretch ratio of 7 would have an MD stretch parameter of 6.
In reference to simultaneous biaxial stretching, the term xe2x80x9cproportional stretch profilexe2x80x9d is a stretch profile in which the ratio of the first direction stretch parameter to the second direction stretch parameter is kept substantially constant throughout the stretch process. A particular example of this would be the case where the ratio of the MD stretch parameter to the TD stretch parameter is kept substantially constant throughout the stretch process. As illustrated in FIG. 1, a plot of MD stretch parameter (y-axis) vs. TD stretch parameter (x-axis) for a proportional stretch profile provides a straight line 10 between the point 12 representing zero MD stretch parameter (or an MD stretch ratio of 1) and zero TD stretch parameter (or a TD stretch ratio of 1) to the point 14 representing the final MD stretch parameter and the final TD stretch parameter. For a proportional stretch profile, this line 10 is straight whether the final MD and TD stretch parameters are equal (a xe2x80x9cbalancedxe2x80x9d stretch) or unequal. Also identified on FIG. 1 is the area A under the curve 10 for the proportional stretch profile.
The term xe2x80x9cMD overbiasxe2x80x9d refers to a stretch profile in which the MD stretch ratio during a significant portion of the stretching process is greater than it would be for the proportional stretch profile having the same final MD and TD stretch ratios. One representative MD overbias curve is represented as 16 on FIG. 1. Another way to identify an overbias stretch profile is that the area B under the curve 16 is greater than area A for a proportional stretch profile ending at the same final MD and TD stretch parameter values. An MD overbias profile does not necessarily exclude having some portion of the profile under the proportional stretch profile line 10.
When many films are stretched uniaxially or biaxially at a temperature below the melting point of the polymer, particularly at a temperature below the line drawing temperature of the film, the film stretches non-uniformly, and a clear boundary is formed between stretched and unstretched parts. This phenomenon is referred to as necking or line drawing. Substantially the entire film is stretched uniformly when the film is stretched to a sufficiently high degree. The stretch ratio at which this occurs is referred to as the xe2x80x9cnatural stretch ratioxe2x80x9d or xe2x80x9cnatural draw ratio.xe2x80x9d The necking phenomenon and the effect of natural stretch ratio is discussed, for example, in U.S. Pat. Nos. 3,903,234; 3,995,007; and 4,335,069. Most discussions of natural draw ratio for biaxial orientation processes are with respect to sequential stretching processes. In such a process, for either a natural draw ratio in the first stretching direction or a natural draw ratio in the second stretching direction, the natural draw ratio in question is substantially analogous to that for a uniaxial stretch. When stretching is done at temperatures near the melting point, or when simultaneous equal biaxial stretching (also referred to a square stretching) is performed, the necking phenomena can be less pronounced, resulting in stretched areas having different local stretch ratios, rather than strictly stretched and unstretched parts. In such a situation, and in any simultaneous biaxial stretching process, the xe2x80x9cnatural stretch ratioxe2x80x9d for a given direction is defined as that global stretch ratio at which the relative standard deviation of the local stretch ratios measured at a plurality of locations upon the film is below about 15%. Stretching above the natural stretch ratio is widely understood to provide significantly more uniform properties or characteristics such as thickness, tensile strength, and modulus of elasticity. For any given film and stretch conditions, the natural stretch ratio is determined by factors such as the polymer composition, morphology due to cast web quenching conditions and the like, and temperature and rate of stretching. Furthermore, for biaxially stretched films, the natural stretch ratio in one direction will be affected by the stretch conditions, including final stretch ratio, in the other direction. Thus, there may be said to be a natural stretch ratio in one direction given a fixed stretch ratio in the other, or, alternatively, there may be said to be a pair of stretch ratios (one in MD and one in TD) which result in the level of local stretch uniformity by which the natural stretch ratio is defined above.